Fullerenes were discovered in 1985 by Curl, Kroto, and Smalley, and carbon nanotubes were discovered a few years later by Sumio Iijima in 1991. See Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F. and Smalley, R. E. “C60: Buckminsterfullerene”, Nature, 318, 162-163 (1985) and Iijima, “Helical Microtubules of Graphitic Carbon”, Nature, 354(7), 56-58 (1991). Since these discoveries, much research has been devoted to learning more about the physical and chemical properties of carbon nanotube materials, as well as potential applications for these materials. However, research has been limited by the lack of a practical method for producing high quality SWNT on a large scale and at a reasonable cost.
The most common methods for the preparation of single wall carbon nanotube material include laser evaporation, electric arc discharge, and chemical vapor deposition methods. However, each of the techniques developed to date has various shortcomings for the large-scale production of high purity SWNT material.
Laser evaporation of graphite has been used to produce SWNT material. In such a process, a laser is used to vaporize a heated carbon target that has been treated with a catalyst metal. In Guo, T. et al., Chem. Physics Letters 243, 49 (1995), and Bandow, S. et al., Physical Review Letters, 80(17), 3779-3782 (1998), a graphite rod having cobalt or nickel dispersed throughout is placed in a quartz tube filled with about 500 Torr of argon, followed by heating to 1200° C. A laser is then focused on the upstream side of the quartz tube from the tip to heat the carbon rod and evaporate it. Carbon nanotubes are then collected on the downstream side of the quartz tube. Laser ablation of a heated target is reported in Thess, A. et al., Science, 273, 483-487 (1996), where a laser is used to vaporize a heated carbon target that has been treated with a catalyst metal such as nickel, cobalt, iron, or mixtures thereof.
An electric arc discharge method for preparation of SWNT has been reported in Iijima, Nature, 354(7), 56-58 (1991) or Wang et al., Fullerene Sci. Technol., 4, 1027 (1996), for example. In this method, carbon graphite is vaporized by direct-current electric arc discharge, carried out using two graphite electrodes in an argon atmosphere at approximately 100 Torr. SWNT grow on the surface of the cathode.
Chemical vapor deposition approaches for growing SWNT material typically use methane, carbon monoxide, ethylene or other hydrocarbons at high temperatures with a catalyst. Chemical vapor deposition of an aerogel supported Fe/Mo catalyst at 850-1000° C. is reported, for example, in J. Kong, A. M. Cassell, and H. Dai, Chemical Physics Letters, 292, 567-574 (1998) and Su, M., Zheng, B., Liu, J., Chemical Physics Letters, 322, 321-326 (2000). The chemical vapor deposition of methane over well-dispersed metal particles supported on MgO at 1000° C. is reported in Colomer, J.-F., et al., Chemical Physics Letters, 317, 83-89 (2000). In Japanese Patent No. 3007983, a CVD process for production of carbon nanotubes is reported where a hydrocarbon is decomposed at 800-1200° C. in a reactor containing a catalyst comprising molybdenum or a metal molybdenum-containing material. In addition to the above methods, a carbon fiber gaseous phase growth method has been reported in WO 89/07163, where ethylene and propane, with hyperfine metal particles are inducted to produce SWNT at 550-850° C.
WO 00/17102 discloses that SWNT material can be prepared by catalytic decomposition of a carbon-containing compound, (e.g., carbon monoxide and ethylene), over a supported metal catalyst at initial temperatures of about 700° C. to about 1200° C., preferably an initial temperature of 850° C. WO 00/17102 asserts that “the mass yield of SWNT is temperature dependent, with the yield increasing with increasing temperature” at page 13, lines 18-19.
EP 1,061,041 teaches a low-temperature thermal chemical vapor deposition apparatus and method of synthesizing SWNTs using the apparatus. This apparatus has a first region, maintained at a temperature of 700° C. to 1000° C., and a second region maintained at 450-650° C. In this process, a metal catalyst is used with a hydrocarbon gas having 1-20 carbon atoms as the carbon source, preferably acetylene or ethylene.
The methods developed to produce SWNT, however, have various shortcomings. Such methods for preparing carbon nanotubes are not only expensive, but also fail to provide carbon nanotubes in high yields or in a cost effective manner. Moreover, the current methods in the art often produce a material of low purity and/or low quality. In current prior processes, SWNT is typically produced by high temperature processes, often with concomitant formation of significant amounts of amorphous carbon or non-nanotube carbon, which typically results in low yields and requires extensive purification steps. The purification techniques themselves often contribute to the low yields by causing damage or breakage of the carbon nanotubes. As a result, the current processes for making SWNT material are expensive and generally prohibit large-scale production of SWNT material.
Additionally, in the methods known in the art, SWNT growth occurs only for a relatively short period of time. This relatively short period of SWNT growth can largely be attributed to the formation of carbon residue from the hydrocarbon gas used in the CVD process. The carbon residue may form detrimentally on the surface area of catalyst that is not presently growing SWNT, which, once becoming covered with carbon residue, cannot grow new SWNT. Replenishing the catalyst supply is then needed to achieve further SWNT growth. However, replenishing the catalyst supply is both expensive and time consuming.
Thus, what is needed in the art is a process for the production of SWNT that allows SWNT growth to be continuous in the sense that the SWNT can grow for significantly longer periods of time without the need to replenish the catalyst. Under such a system, the SWNT yield, defined as the amount of SWNT growth per gram of catalyst, would be dramatically increased. The process should preferably produce high quality SWNT material that allows for the high-yield SWNT growth to continue to grow past the period of time when the typical SWNT is normally prevented. The new processes and new apparatus for producing SWNT disclosed in this invention provide a new geometry/arrangement for SWNT growth that answers such a need.